The invention relates to the field of photoacoustics, and more particularly to thermoacoustic or thermal wave microscopy for the microscopic examination of materials, expecially bulk solids, by photoacoustic methods.
The photoacoustic effect was first discovered about one hundred years ago. The photoacoustic effect arises when intensity-modulated light, or other form of electromagnetic radiation, is absorbed by a sample, thereby exciting energy levels within the sample. These levels deexcite generally through a non-radiative or heat-producing process. Thus the absorption of intensity-modulated electromagnetic radiation at any point in the sample results in a periodic localized heating of the sample medium.
The field of photoacoustics has undergone extensive development in the past few years, particularly in the area of photoacoustic spectroscopy. Applicant's prior U.S. patents, U.S. Pat. Nos. 3,948,345 and 4,028,932, disclose a photoacoustic method for spectroscopically analyzing solid substances and a photoacoustic cell. Also see applicant's paper A. Rosencwaig, Optoacoustic Spectroscopy and Detection, (Y.H. Pao, ed.), Ch. 8, Academic Press, N.Y., 1977.
Photoacoustic studies on gaseous samples are generally performed using a microphone as a detector. The gaseous sample is contained in an acoustically sealed chamber which also contains a sensitive microphone. Light, chopped or intensity-modulated at a frequency in the range 20-10,000 Hz enters the chamber through a non-absorbing window. If the gas sample absorbs any of the incident photons, energy levels are excited in the gas molecules. When these levels deexcite, some or all of the energy is transferred through collisional processes to kinetic energy of the molecules. The gas thus undergoes a periodic heating as a result of the absorption of the intensity-modulated light, creating thermal waves. This periodic heating in turn results in a periodic pressure rise in the gas, and these pressure fluctuations or acoustic waves are then detected by the microphone.
A great deal of the current work in photoacoustic spectroscopy involves the investigation of non-gaseous materials, such as powders, that are highly light scattering. A similar gas-microphone technique is used. The powder sample is placed in an acoustically sealed cell which also contains a nonabsorbing gas and a sensitive microphone. The incident light is intensity modulated or chopped at a frequency in the range of 20-10,000 Hz. If the powder sample absorbs any incident photons, energy levels in the sample are excited. Deexcitation through non-radiative processes will produce internal heating of the sample. This, in turn, will result in a periodic heat flow or thermal wave from the sample to the surrounding gas. The gas layer near the solid particles undergoes a periodic heating from this heat flow, and this results in an acoustic pressure signal or acoustic wave in the cell that is detected by the microphone. Although indirect, the gas microphone technique is fairly sensitive for measuring the internal heating of a powder sample because of the large surface:volume ratio of the powder.
Photoacoustics of a bulk solid sample are often best measured using the piezoelectric method. The gas-microphone method usually does not work well because of the low surface:volume ratio for the solid and the consequent reduction in heat transfer from the sample to the gas in the cell. The amount of internal heating in the bulk solid is efficiently measured by a piezoelectric transducer in direct contact with the sample. The piezoelectric transducer detects an acoustic wave generated in the solid by the thermal wave produced through the absorption of incident light. A lead zirconate titanate crystal or PZT is a suitable piezoelectric. The sample is illuminated with intensity-modulated light, with modulation frequencies into the megahertz range if desired because of the wide bandwidth response of the piezoelectric detector.
Recently there have been some experiments related to the possibility of performing photoacoustics on a microscopic scale. Von Gutfeld and Melcher, "20-MHz Acoustic Waves From Pulsed Thermo-elastic Expansions of Constrained Surfaces", Appl. Phys, Lett., Vol. 30, No. 6, p. 257, Mar. 15, 1977, describe the generation of acoustic waves in a material by focusing a pulsed laser beam onto the material, and use a piezoelectric detector. However, they operate at a very high frequency of 20 MHz to produce acoustic waves. The surface is constrained to enhance the signal. Wong et al, "Surface and Subsurface Structure of Solids by Laser Photoacoustic Spectroscopy", Appl. Phys. Lett., 32 (9), May 1, 1978, p. 538 describes a preliminary study of photoacoustic microscopy using a gas-microphone detector system. Wickramasinghe et al "Photoacoustics on a Microscopic Scale", to be published in Appl. Phys. Lett., Dec. 1, 1978, describes the modification of a transmission acoustic microscope by replacing an input acoustic lens with an optical counterpart, a focused pulsed laser. The system operates at a very high fixed frequency of 840 MHz. Hordvik and Schlossberg, "Photoacoustic Technique for Determining Optical Absorption Coefficients in Solids", Applied Optics, Vol. 16, No. 1, January 1977, p. 101, describes a photoacoustic method using a contact transducer detector to measure the absorption coefficients of solids. White, J. Appl. Phys., 34, 3359 (1963) shows the generation of elastic waves by very high frequency surface heating. Callis, "The Calorimetric Detection of Excited States", J. Research N.B.S., Vol. 80A, No. 3, May-June 1976, p. 413 describes a piezoelectric calorimeter. Farrow et al, "Piezoelectric Detection of Photoacoustic Signals", Applied Optics, Vol. 17, No. 7, Apr. 1, 1978, p. 1093, describe the use of piezoelectric detectors instead of microphone detectors to measure optically generated acoustic signals in solids.
None of the prior art fully explores the physical mechanisms and potentials of photoacoustic microscopy. The prior art does not go to the underlying basis of operation, does not teach how to perform thermoacoustic microscopy with photoacoustics, and does not teach the many uses for photoacoustic microscopy.
There is a great need in many industries, particularly the semiconductor industry, to rapidly scan a bulk solid material on a microscopic scale to determine subsurface properties and to do so in a nondestructive manner. Information about subsurface structures, material changes, and competing energy-conversion properties is needed, and in particular the ability to obtain a depth-profile at various selected depths would be highly advantageous for quality control of devices early in the manufacturing process. Present optical techniques provide information about surface properties. Photoacoustic spectroscopy provides information as a function of wavelength as the source wavelength is varied, i.e. photoacoustically generated spectra, and only for the aggregate material, not on a microscopic scale.
It is an object of the invention to provide non-destructive measurements of surface and subsurface properties of a bulk solid on a microscopic scale.
It is also an object of the invention to provide non-destructive measurements of surface and subsurface features of a bulk solid on a microscopic scale through the interaction of photoacoustically generated thermal waves with the features.
It is another object of the invention to detect the presence of, and perform measurements on, fluorescent species, photochemical processes, and photovoltaic processes in a bulk solid on a microscopic scale.
It is a further object of the invention to provide a depth-profile of a bulk solid at various selected depths on a microscopic scale.